1. Field of the Invention
This invention relates to a sewing machine including a cassette mount to which a thread cassette is attached and a thread tension control program therefor, and more particularly to a technique for reliably and desirably adjusting the tension of a thread drawn from the thread cassette when the thread cassette is attached to and detached from the cassette mount.
2. Description of the Related Art
Sewing machines have commercially been provided including a cassette mount to which a thread cassette enclosing a thread spool or bobbin is detachably attached in order that a thread drawn from the thread spool may be used as an upper thread. The thread drawn from the thread cassette attached to the cassette mount is placed between a pair of thread tension discs. The thread extending from the thread tension discs is further placed on a thread take-up. The thread extending from the thread take-up is passed through an eye of a sewing needle so that the thread is set.
In one type of the above-described sewing machines, the thread drawn from the thread cassette is automatically placed between the thread tension discs. However, in order that sewing may be started, the thread tension discs need to be closed so that the thread is held therebetween. Furthermore, in order that the thread cassette may be detached from the cassette mount, the thread tension discs need to be opened so that the thread is released from the held state.
In view of the aforesaid requirement, the assignee of the present application filed a patent application for a sewing machine including a thread tension disc operating mechanism operated mechanically in synchronization with attachment of the thread cassette. This Japanese patent application was published on Jul. 10, 2002 under the publication number JP-A-2002-191884. In the disclosed sewing machine, a pair of thread tension discs are held in a closed state when a thread cassette is absent in a cassette mount. When the thread cassette is inserted into the cassette mount, the thread tension disc operating mechanism is firstly operated to open the thread tension discs.
The thread drawn from the thread cassette is subsequently placed between the thread tension discs, and simultaneously, the discs are closed such that the thread is held between the discs. Furthermore, when the thread cassette is detached from the cassette mount, the above-described operation is performed in a reverse sequence by the thread tension disc operating mechanism so that the thread is released from the thread tension discs.
U.S. Pat. No. 3,749,039 to Russell A. Fritts discloses a sewing machine including a rotatable guide member provided at the thread tension disc side. The guide member corresponds to a part of the aforesaid thread tension disc operating mechanism and is rotated by attaching and detaching the thread cassette to and from the cassette mount, whereby the thread tension discs are opened and closed. Furthermore, the prior art has provided a technique for opening a pair of thread tension discs by raising a presser foot pressing cloth. Additionally, a lever may be provided which is operated to detach the thread cassette from the thread attaching portion.
In the aforesaid sewing machine disclosed in JP-A-2002-191884, however, the thread tension disc operating mechanism and the thread tension discs are operated in response to attachment or detachment of the thread cassette to or from the cassette mount. The force operating the thread tension disc operating mechanism and the thread tension discs needs to be exerted by an operator. Since this increases load of the operator, the thread cassette cannot be attached to and detached from the cassette mount smoothly. Furthermore, the thread tension discs closed need to be opened and thereafter re-closed when the thread cassette is attached to the cassette mount. However, it is difficult to obtain proper timings for the opening and closure of the thread tension discs. Accordingly, the thread extending from the thread cassette cannot sometimes be caught by the thread tension discs reliably. The thread cassette needs to be re-attached to the cassette mount when the thread has not been caught by the thread tension discs. This would result in troublesomeness.
Therefore, an object of the present invention is to provide a sewing machine in which the load imposed on the operator can be reduced with improved smoothness in the attachment and detachment of the thread cassette when the thread cassette is attached to and detached from the cassette mount, and the tension of the thread drawn from the thread cassette can be adjusted reliably and desirably, and to provide a thread tension control program which can be applied to the sewing machine.
The present invention provides a sewing machine comprising a thread cassette having a thread accommodating cavity in which a supply of thread is accommodated, a cassette mount to which the thread cassette is detachably attached, a thread tensioner adjusting a tension of the thread drawn from the thread cassette, a detachment operating member operated so that the thread cassette is detached from the cassette mount, and a thread tension control controlling the thread tensioner so that the thread is set at a predetermined tension when the detachment operating member has been operated.
The tension of thread drawn from the thread cassette is adjusted by the thread tensioner in the above-described sewing machine. The detachment operating member is operated so that the thread cassette is allowed to be detached from the cassette mount. Upon operation of the detachment operating member, the thread tension control controls the thread tensioner so that the predetermined tension is applied to the thread. Accordingly, the load imposed on the operator can be reduced with improvement of the smoothness in the attachment and detachment of the thread cassette when the operator detaches the thread cassette from the cassette mount. Consequently, the tension of the thread drawn from the thread cassette can be adjusted reliably and desirably.
In a preferred form, the thread tension control opens the thread tensioner when the detachment operating member has been operated. In another preferred form, the thread tensioner applies no tension to the thread when the thread tensioner is opened. Furthermore, the sewing machine preferably further comprises a thread tensioner opening mechanism operated in synchronization with the detachment operating member and opening the thread tensioner when the detachment operating member is operated.
The invention also provides a sewing machine comprising a thread cassette having a thread accommodating cavity in which a supply of thread is stored, a cassette mount to which the thread cassette is detachably attached, a thread tensioner adjusting a tension of the thread drawn from the thread cassette, a cassette detector detecting the thread cassette having been attached to the cassette mount, and a thread tension control controlling the thread tensioner so that the thread is tensioned a predetermined period of time after detection of the thread cassette by the cassette detector.
The tension of thread drawn from the thread cassette is adjusted by the thread tensioner also in the above-described sewing machine. When having been attached to the cassette mount, the thread cassette is detected by the cassette detector. The thread tensioner is controlled by the thread tension control the predetermined period of time after detection of the thread cassette by the cassette detector, whereupon the thread drawn from the thread cassette is tensioned. Thus, the thread drawn from the thread cassette is tensioned the predetermined period of time after detection of the thread cassette by the cassette detector. Consequently, the thread can reliably be caught and held by the thread tensioner.
The aforesaid predetermined period of time is preferably set at a value not more than 30 msec.